1. Field of the Invention
Embodiments of the present invention relate to tracking of concealed non-metallic underground utilities using a pipe and cable locator outfitted with ground penetrating radar and an inertial navigation device.
2. Discussion of Related Art
Utility conduits are often buried underground and not readily accessible. It is often necessary to locate these concealed utility conduits in order to repair and replace them. It is also important to know the location of utility conduits in order to avoid them while excavating an area. Examples of hidden utility conduits include pipelines for gas, sewage, or water and cables for telephone, television or power.
There are various ways to locate concealed objects, for example, using pipe and cable locators, also known as line locators. Conventional line locators are appropriate when seeking electrically conductive objects, such as metallic pipelines and cables. Line locators may also be used for finding non-electrically conducting conduits when the conduit is marked with a conducting trace wire or trace tape buried along the conduit. The process of applying an AC signal to the conductor at an accessible point and detecting the resulting electromagnetic radiation is well known in the art. When an AC signal is applied, the conductor acts as an antenna radiating an electromagnetic field along its entire length. A fully digital implementation of an electromagnetic line locator is disclosed in U.S. patent application Ser. No. 10/622,376, “Method and Apparatus for Digital Detection of Electromagnetic Signal Strength and Signal Direction in Metallic Pipes and Cables”, by James W. Waite and Johan D. Överby.
A line locator used above ground detects electromagnetic emissions from conductors underground. A disadvantage with relying solely on the line locator device is that it may fail to identify and distinguish among various utility conduits and conductors, especially non-conductive lines, such as, for example, gas lines, fiber optic lines and plastic water lines when not marked with trace wires.
For some time, ground penetrating radar (GPR) systems have been used for utility locating applications. Several commercial systems exist, an example of which is shown in FIG. 1. All such systems present to the user a fairly complex grey-scale or color image of the radar pulse echo amplitude in a 2-D map of depth vs. horizontal ground position. This type of display, an example of which is shown in FIG. 2, is useful in survey and mapping applications, but provides far too much information for the ordinary user trained in line location techniques. In such existing systems, the results that signify the presence of underground utility lines are often not available until after post processing the image on a computer.
To accomplish an underground survey, the recommended mode of operation of such prior art GPR systems is depicted in FIG. 3. Dipole antenna assembly 301 is moved across the ground in the direction of path 304. Transmit “bowtie” antenna 302 couples a radar pulse 307 into the ground and receive bowtie antenna 303 receives a reflected signal 308. Utility lines 309 and 310 represent discontinuities in the dielectric constant of the soil medium, thus reflections occur and the lines can be discriminated from the background. Sequence of traces 311 represent an example of the radar return from lines 309, 310, and by viewing the entire sequence one can deduce the presence of an underground object. The aggregate image often takes the form of the series of overlapping hyperbolas seen in FIG. 2. Subsequent path directions 305, 306 are surveyed to fill in a large matrix of radar echo data. For test fields where there is no a priori knowledge of the direction of the utility lines 309, 310, a second set of paths are mapped which are orthogonal to paths 304, 305, 306, resulting in a 2-D grid of path directions.
From this collected data a post-processing operation can take place that determines the position of the underground utilities. The result of this operation is shown in FIG. 4. Though this particular pattern of utility lines is very complex, they are all denoted as having the same diameter, since for GPR systems, the object size (pipe or cable diameter) cannot normally be deduced. An exception to this rule is when the object size is greater than ¼ wavelength of the radar frequency. For common 500 Mhz GPR systems, lines less than six inches in diameter have a similar signal return and thus the size cannot be discriminated. At 200 MHz (which allows a deeper penetration of the radar signal, facilitating deeper detection of utility lines), the limit for object size detection is 15 inches.
The lack of a utility line diameter estimate is not a critical problem for the locate technician, since this feature is not available even when the line is metallic. A far larger problem is that existing GPR systems focus on collection of data, with subsequent post-processing, and not the detection and tracking in real-time of an individual utility line. Electromagnetic (EM) line locator user interfaces are designed for infrequent use. The locate technician must be able to pick up the instrument after sporadic use and be able to track a specific line though an unfamiliar area. The line location is often marked with paint and must have accuracy within accepted (and sometimes legal) guidelines. The images of the reflected GPR signals shown in FIG. 2, though appearing static in the figure, constantly change when the user is walking the locator down the line. Features are often inconsistent and unrecognizable when these maps are presented in real-time. Using conventional methods without off-line image processing, centerline errors that are only a few centimeters for electromagnetically traced lines would not be achievable for a GPR tracked line.
Depth of the target utility line is an important parameter in any locating task. This is because the locate operation often precedes a digging operation, and the result of the locate determines how deep a backhoe operator can dig without impacting the utility line. To achieve acceptable depth accuracy, prior art GPR systems must be calibrated by locating a known underground object on the same test site (since soil conditions must be identical between the calibration and the locate), and then digging down to the object to determine the exact depth. Without this absolute depth confirmation, GPR systems rely on a user-specified choice of the dielectric constant of the soil at the test site. This is often difficult at best, and depth determinations are subject to 20% tolerances in this scenario.
In light of the foregoing description, it would be desirable to develop a dual-mode line locator that simultaneously offers both metallic (electromagnetic) and non-metallic location methods, particularly for the real-time location of the line's centerline and depth. Since ground penetrating radar (GPR) is an accepted tool for the survey of underground buried objects, it would be of great advantage to algorithmically process radar range data obtained from such a GPR front-end and present a simple user interface display, enabling an operator familiar with conventional electromagnetic line tracing methods to trace non-conductive lines. It would be of further advantage if the dual mode locator achieved equivalent centerline and depth accuracies for both metallic and non-metallic utility lines, in real-time as a locate technician is walking the line and marking it with paint.